Re: [Qemu-ppc] [Qemu-devel] [PATCH v4 06/15] target/ppc: remove xer split-out flags(so, ov, ca)

[Nikunj A Dadhania]

Nikunj A Dadhania <address@hidden> writes:

> Richard Henderson <address@hidden> writes:
>
>> On 02/24/2017 01:58 PM, David Gibson wrote:
>>> On Fri, Feb 24, 2017 at 06:18:22AM +0530, Nikunj A Dadhania wrote:
>>>> Richard Henderson <address@hidden> writes:
>>>>
>>>>> On 02/24/2017 06:56 AM, Nikunj A Dadhania wrote:
>>>>>> Now get rid all the split out variables so, ca, ov. After this patch,
>>>>>> all the bits are stored in CPUPPCState::xer at appropriate places.
>>>>>>
>>>>>> Signed-off-by: Nikunj A Dadhania <address@hidden>
>>>>>> ---
>>>>>>  target/ppc/cpu.c        |   8 +---
>>>>>>  target/ppc/cpu.h        |  26 ++++++------
>>>>>>  target/ppc/int_helper.c |  12 +++---
>>>>>>  target/ppc/translate.c  | 106 
>>>>>> +++++++++++++++++++++++++-----------------------
>>>>>>  4 files changed, 78 insertions(+), 74 deletions(-)
>>>>>
>>>>> I do not think this is a good direction to take this.
>>>>
>>>> Hmm, any particular reason?
>>>
>>> Right, I suggested this, but based only a suspicion that the split
>>> variables weren't worth the complexity.  I'm happy to be corrected by
>>> someone with better knowledge of TCG, but it'd be nice to know why.
>>
>> Normally we're interested in minimizing the size of the generated code, 
>> delaying computation until we can show it being used.
>>
>> Now, ppc is a bit different from other targets (which might compute overflow 
>> for any addition insn) in that it only computes overflow when someone asks 
>> for 
>> it.  Moreover, it's fairly rare for the addo/subo/nego instructions to
>> be used.
>
>> Therefore, I'm not 100% sure what the "best" solution is.
>
> Agreed, with that logic, wont it be more efficient to move the OV/CA
> updationg to respective callers, and when xer_read/write happens, its
> just one tcg_ops.

BTW, i haven't seen remarkable difference in the boot time after this
change.

Regards
Nikunj